Vector Based Art Software

Adobe Illustrator - $599, Free 30 Day Trial
Illustrator is preferred if you are drawing in vector format. You can import vector art originally generated in Flash, which is handy if you want to put something on a t-shirt.

Inkscape - FREE
Inkscape is a free and open source software vector graphics editor. Its goal is to implement full support for the Scalable Vector Graphics 1.1 standard.